Some radiation-curable, e.g., UV-curable, jetting inks are liquid at room temperature. To ensure correct jetting viscosity, these liquid radiation-curable inks are often jetted above room temperature, e.g., 30° C. or more, e.g., 40° C. Such inks can be jetted onto substantially non-porous substrates, e.g., plastic pen barrels or circuit boards, or porous substrates. When such liquid radiation-curable inks are jetted onto a substrate, e.g., paper or plastic, to form an image, phenomena such as bleed-through, pinhole wetting and fisheyes due to the wetting characteristics of the liquid can result in inadequate ink coverage and overall poor print quality. One solution that is often used to reduce wicking is to treat the substrate to make it less porous. However, some inks do not perform well with such treatments. Another solution to minimizing wicking and bleed-through is to rapidly surface cure the ink, but often this does not completely eliminate wicking and bleed-through, and can require cumbersome and expensive equipment.
“Hybrid-F” radiation-curable jetting inks, i.e., those that polymerize by radical and/or cationic mechanisms to give polymer networks, are often described as “semi-solid inks,” and are more viscous at room temperature than at jetting temperature. Hybrid-F inks are available from Aellora™, e.g., under the tradename VistaSpec™ HB. Typically, these inks are jetted at elevated temperatures, e.g., above 60° C. or above 65° C., to lower ink viscosity to an appropriate jetting viscosity. After jetting hybrid-F ink, e.g., through a piezoelectric drop-on-demand inkjet printhead, ink viscosity rapidly increases as the ink cools on contact with the substrate. Once cooled to about room temperature, the hybrid-F ink does not flow without shear, allowing “wet-on-wet” printing without intermediate curing stages. Since the hybrid-F ink does not substantially flow at room temperature, wetting defects can be reduced, often reducing or eliminating the need for substrate surface treatments.